1. Field of the Invention
The present invention relates to an inexpensive high-strength composition iron powder used as a raw material powder of a sintered part, and a sintered part made from the high-strength composition iron powder.
2. Description of the Related Art
Sintered parts obtained by press-forming metal powders into green compacts and sintering the green compacts are used as automobile parts such as synchronizer hubs and vane pump rotors, for example. Since automobile parts are required to achieve weight reduction to lower the fuel consumption, they are also required to achieve a higher strength. To satisfy such a requirement, alloyed steel powders containing Ni and Mo as the reinforcing elements are usually used as the metal powders.
One example of such an alloyed steel powder is an iron-based 0.6% carbon, 0.5% molybdenum alloyed powder (carbon-molybdenum material) prepared by blending an iron powder, a lubricant, ferromolybdenum, and graphite disclosed in U.S. Pat. No. 5,997,805. The '805 document teaches that when this carbon molybdenum alloyed powder is compacted into test rings under a compacting pressure of about 6.1×108 Pa, heated to sinter, and then subjected to high-density secondary forming operation at a pressure of 6.1×108 Pa, a density greater than 7.5 g/cm3 is achieved, which shows clear improvements in dynamic properties from that achieved by the conventional process.
Japanese Unexamined Patent Application Publication No. 2007-23318 discloses alloyed steel powders, namely, mixed powders prepared by mixing a pure iron powder with a prealloyed steel powder containing 0.5% Ni, 0.5% Mo, and 0.2% Mn serving as alloy components at a variety of mixing ratios, and adding a graphite powder and a Cu powder to the resulting mixture. The mixed powders are press-formed into round bar-shaped test pieces under a pressure of 6 ton/cm2. The test pieces are sintered, hot-forged, and evaluated in terms of strength properties such as tensile strength and self-aligning properties during assembly of the sintered parts, the results of which are disclosed in the '318 document.
However, recent price surge of alloying elements, in particular, Ni and Mo, has let to an increase in manufacture cost of sintered parts produced by using starting material powders containing Ni and Mo. Thus, an inexpensive high-strength steel powder that contains alloying elements that replace Ni and Mo is desired.